The morphology and function of the renal tubules have changed during the phylogenetic history of the vertebrates in order to adapt to a variety of environments. We intend to define a functional evolution of the nephron, which initially emerged as a primitive excretory device and culminated in the highly specialized mammalian kidney. During the current year, we found: 1) angiotensin and arginine vasotocin caused vasopressor responses in the aglomerular toadfish, Opsanus tau, but these hormones did not elicit diuresis, suggesting that the diuresis and natriuresis shown in the glomerular teleosts may largely be due to increased glomerular filtration rate secondary to increased dorsal aortic pressure. 2) The proximal tubule has low transtubular potential differences (PD), oriented negative in the lumen. On the other hand, lumen positive PD was found in the collecting tubule from toadfish and the distal tubule from freshwater catfish, which were blocked by ouabain and furosemide, respectively. 3) The function of the "vascular receptor" appears to exist in the granulated cells of the aglomerular kidney and responds to hemorrhage and hypotension by causing renin release. We will continue to determine transport properties of renal tubules and their permeability to water and ions in the marine and freshwater fishes by using both in vivo and in vitro isolated perfused tubles, and effects of various hormones and transport inhibitors on them. We also plan to define the role of the renin-angiotensin system in cardiovascular and renal homeostasis in primitive animals. Comparative studies will provide new insight and better understanding into the underlying mechanisms operating in man and other mammals.